Enhancement of methane production from 1-hexadecene by additional electron donors

1-Hexadecene-contaminated wastewater is produced in oil refineries and can be treated in methanogenic bioreactors, although generally at low conversion rates. In this study, a microbial culture able to degrade 1-hexadecene was enriched, and different stimulation strategies were tested for enhancing 1-hexadecene conversion to methane. Seven and three times faster methane production was obtained in cultures stimulated with yeast extract or lactate, respectively, while cultures amended with crotonate lost the ability to degrade 1-hexadecene. Methane production from 1-hexadecene was not enhanced by the addition of extra hydrogenotrophic methanogens. Bacteria closely related to Syntrophus and Smithella were detected in 1-hexadecene-degrading cultures, but not in the ones amended with crotonate, which suggests the involvement of these bacteria in 1-hexadecene degradation. Genes coding for alkylsuccinate synthase alpha-subunit were detected in cultures degrading 1-hexadecene, indicating that hydrocarbon activation may occur by fumarate addition. These findings are novel and show that methane production from 1-hexadecene is improved by the addition of yeast extract or lactate. These extra electron donors may be considered as a potential bioremediation strategy of oil-contaminated sites with bioenergy generation through methane production.

Biofilm formation and granule properties in anaerobic digestion at high salinity

For the anaerobic biological treatment of saline wastewater, Anaerobic Digestion (AD) is currently a possibility, even though elevated salt concentrations can be a major obstacle. Anaerobic consortia and especially methanogenic archaea are very sensitive to fluctuations in salinity. When working with Upflow Sludge Blanket Reactor (UASB) technology, in which the microorganisms are aggregated and retained in the system as a granular biofilm, high sodium concentration negatively affects aggregation and consequently process performances. In this research, we analysed the structure of the biofilm and granules formed during the anaerobic treatment of high salinity (at 10 and 20 g/L of sodium) synthetic wastewater at lab scale. The acclimated inoculum was able to accomplish high rates of organics removal at all the salinity levels tested. 16S rRNA gene clonal analysis and Fluorescence In Situ Hybridization (FISH) analyses identified the acetoclastic Methanosaeta harundinacea as the key player involved acetate degradation and microbial attachment/granulation. When additional calcium (1 g/L) was added to overcome the negative effect of sodium on microbial aggregation, during the biofilm formation process microbial attachment and acetate degradation decreased. The same result was observed on granules formation: while calcium had a positive effect on granules strength when added to UASB reactors, Methanosaeta filaments were not present and the degradation of the partially acidified substrate was negatively influenced. This research demonstrated the possibility to get granulation at high salinity, bringing to the forefront the importance of a selection towards Methanosaeta cells growing in filamentous form to obtain strong and healthy granules.

Microbial selenium sulfide reduction for selenium recovery from wastewater

Microbial reduction of selenium sulfide (SeS₂) is a key step in a new treatment process to recover selenium from selenate and selenite streams. In this process, selenate is first reduced to selenite, and subsequently selenite is reduced by sulfide and precipitates from the solution as SeS₂. The latter is bio-reduced to elemental selenium and sulfide. Two anaerobic granular sludges (Eerbeek and Emmtec) were tested for their efficiency to reduce commercial crystalline SeS₂. Emmtec sludge had the highest reducing capacity with commercial SeS₂ and was therefore also used for the bioreduction of laboratory synthesized amorphous SeS₂. Synthesized SeS₂ was formed mixing a sulfide solution and effluent containing selenite. With both SeS₂ solids (commercial and synthesized SeS₂), Emmtec sludge produced sulfide and a solid consisting of hexagonal elemental selenium. The crystalline hexagonal structure suggests the absence of biomolecules, which stabilize amorphous selenium bio-particles under comparable process conditions (T=30°C and a pH between 6 and 7). Selenium particles were not attached to the biomass, suggesting an extracellular formation. The results support the feasibility of the bio-reduction process using sulfur for recovering selenium from water.

Effect of conventional chemical treatment on the microbial population in a biofouling layer of reverse osmosis systems

The impact of conventional chemical treatment on initiation and spatiotemporal development of biofilms on reverse osmosis (RO) membranes was investigated in situ using flow cells placed in parallel with the RO system of a full-scale water treatment plant. The flow cells got the same feed (extensively pre-treated fresh surface water) and operational conditions (temperature, pressure and membrane flux) as the full-scale installation. With regular intervals both the full-scale RO membrane modules and the flow cells were cleaned using conventional chemical treatment. For comparison some flow cells were not cleaned. Sampling was done at different time periods of flow cell operation (i.e., 1, 5, 10 and 17 days and 1, 3, 6 and 12 months). The combination of molecular (FISH, DGGE, clone libraries and sequencing) and microscopic (field emission scanning electron, epifluorescence and confocal laser scanning microscopy) techniques made it possible to thoroughly analyze the abundance, composition and 3D architecture of the emerged microbial layers. The results suggest that chemical treatment facilitates initiation and subsequent maturation of biofilm structures on the RO membrane and feed-side spacer surfaces. Biofouling control might be possible only if the cleaning procedures are adapted to effectively remove the (dead) biomass from the RO modules after chemical treatment.

Performance and population analysis of a non-sterile trickle bed reactor inoculated with Caldicellulosiruptor saccharolyticus, a thermophilic hydrogen producer

Non-axenic operation of a 400 L trickle bed reactor inoculated with the thermophile Caldicellulosiruptor saccharolyticus, yielded 2.8 mol H2/mol hexose converted. The reactor was fed with a complex medium with sucrose as the main substrate, continuously flushed with nitrogen gas, and operated at 73 degrees C. The volumetric productivity was 22 mmol H2/(L filterbed h). Acetic acid and lactic acid were the main by-products in the liquid phase. Production of lactic acid occurred when hydrogen partial pressure was elevated above 2% and during suboptimal fermentation conditions that also resulted in the presence of mono- and disaccharides in the effluent. Methane production was negligible. The microbial community was analyzed at two different time points during operation. Initially, other species related to members of the genera Thermoanaerobacterium and Caldicellulosiruptor were present in the reactor. However, these were out-competed by C. saccharolyticus during a period when sucrose was completely used and no saccharides were discharged with the effluent. In general, the use of pure cultures in non-sterile industrial applications is known to be less useful because of contamination. However, our results show that the applied fermentation conditions resulted in a culture of a single dominant organism with excellent hydrogen production characteristics.

Anaerobic microbial LCFA degradation in bioreactors

This paper reviews recent results obtained on long-chain fatty acids (LCFA) anaerobic degradation. Two LCFA were used as model substrates: oleate, a mono-unsaturated LCFA, and palmitate, a saturated LCFA, both abundant in LCFA-rich wastewaters. 16S rRNA gene analysis of sludge samples submitted to continuous oleate- and palmitate-feeding followed by batch degradation of the accumulated LCFA demonstrated that bacterial communities were dominated by members of the Clostridiaceae and Syntrophomonadaceae families. Archaeal populations were mainly comprised of hydrogen-consuming microorganisms belonging to the genus Methanobacterium, and acetate-utilizers from the genera Methanosaeta and Methanosarcina. Enrichment cultures growing on oleate and palmitate, in the absence or presence of sulfate, gave more insight into the major players involved in the degradation of unsaturated and saturated LCFA. Syntrophomonas-related species were identified as predominant microorganisms in all the enrichment cultures. Microorganisms clustering within the family Syntrophobacteraceae were identified in the methanogenic and sulfate-reducing enrichments growing on palmitate. Distinct bacterial consortia were developed in oleate and palmitate enrichments, and observed differences might be related to the different degrees of saturation of these two LCFA. A new obligately syntrophic bacterium, Syntrophomonas zehnderi, was isolated from an oleate-degrading culture and its presence in oleate-degrading sludges detected by 16S rRNA gene cloning and sequencing.

Investigation of microbial communities on reverse osmosis membranes used for process water production

In the present study, the diversity and the phylogenetic affiliation of bacteria in a biofouling layer on reverse osmosis (RO) membranes were determined. Fresh surface water was used as a feed in a membrane-based water purification process. Total DNA was extracted from attached cells from feed spacer, RO membrane and product spacer. Universal primers were used to amplify the bacterial 16S rRNA genes. The biofilm community was analysed by 16S rRNA-gene-targeted denaturing gradient gel electrophoresis (DGGE) and the phylogenetic affiliation was determined by sequence analyses of individual 16S rDNA clones. Using this approach, we found that five distinct bacterial genotypes (Sphingomonas, Beta proteobacterium, Flavobacterium, Nitrosomonas and Sphingobacterium) were dominant genera on surfaces of fouled RO membranes. Moreover, the finding that all five "key players" could be recovered from the cartridge filters of this RO system, which cartridge filters are positioned before the RO membrane, together with literature information where these bacteria are normally encountered, suggests that these microorganisms originate from the feed water rather than from the RO system itself, and represent the fresh water bacteria present in the feed water, despite the fact that the feed water passes an ultrafiltration (UF) membrane (pore size approximately 40 nm), which is able to remove microorganisms to a large extent.

Degradation of methanethiol by methylotrophic methanogenic archaea in a lab-scale upflow anaerobic sludge blanket reactor

In a lab-scale upflow anaerobic sludge blanket reactor inoculated with granular sludge from a full-scale wastewater treatment plant treating paper mill wastewater, methanethiol (MT) was degraded at 30 degrees C to H2S, CO2, and CH4. At a hydraulic retention time of 9 h, a maximum influent concentration of 6 mM MT was applied, corresponding to a volumetric loading rate of 16.5 mmol liter-1 day-1. The archaeal community within the reactor was characterized by anaerobic culturing and denaturing gradient gel electrophoresis analysis, cloning, and sequencing of 16S rRNA genes and quantitative PCR. Initially, MT-fermenting methanogenic archaea related to members of the genus Methanolobus were enriched in the reactor. Later, they were outcompeted by Methanomethylovorans hollandica, which was detected in aggregates but not inside the granules that originated from the inoculum, the microbial composition of which remained fairly unchanged. Possibly other species within the Methanosarcinacaea also contributed to the fermentation of MT, but they were not enriched by serial dilution in liquid media. The archaeal community within the granules, which was dominated by Methanobacterium beijingense, did not change substantially during the reactor operation. Some of the species related to Methanomethylovorans hollandica were enriched by serial dilutions, but their growth rates were very low. Interestingly, the enrichments could be sustained only in the presence of MT and did not utilize any of the other typical substrates for methylotrophic methanogens, such as methanol, methyl amine, or dimethylsulfide.

Benzene Degradation Coupled with Chlorate Reduction in a Soil Column Study

Perchlorate and chlorate are electron acceptors that during reduction result in the formation of molecular oxygen. The produced oxygen can be used for activation of anaerobic persistent pollutants, like benzene. In this study chlorate was tested as potential electron acceptor to stimulate benzene degradation in anoxic polluted soil column. A chlorate amended benzene polluted soil column was operated over a period of 500 days. Benzene was immediately degraded in the column after start up, and benzene removal recovered completely after omission of chlorate or a too high influent chlorate concentration (22 mM). Mass balance calculations showed that per mole of benzene five mole of chlorate were reduced. At the end of the experiment higher loading rates were applied to measure the maximal benzene degradation rate in this system; a breakthrough of benzene was not observed. The average benzene degradation rate over this period was 31 micromol l(-1) h(-1) with a maximal of 78 micromol l(-1) h(-1). The high degradation rate and the necessity of chlorate indicate that oxygen produced during chlorate reduction indeed is used for the activation of benzene. This is the first column study where benzene biodegradation at a high rate coupled with anaerobic chlorate reduction is observed.

Methanomethylovorans thermophila sp. nov., a thermophilic, methylotrophic methanogen from an anaerobic reactor fed with methanol

A novel thermophilic, obligately methylotrophic, methanogenic archaeon, strain L2FAWT, was isolated from a thermophilic laboratory-scale upflow anaerobic sludge blanket reactor fed with methanol as the carbon and energy source. Cells of strain L2FAWT were non-motile, irregular cocci, 0·7–1·5 μm in diameter and usually occurred singly (sometimes forming clusters of two or four cells). The cells stained Gram-negative and lysed immediately in 0·1 % (w/v) SDS. Growth was inhibited by chloramphenicol and tetracycline, but not by penicillin, bacitracin, spectinomycin, vancomycin or kanamycin. Methanol and mono-, di- and trimethylamine were used as substrates, but H2/CO2, formate, acetate, propanol, dimethyl sulfide and methanethiol were not. The temperature range for growth was 42–58 °C, with an optimum at 50 °C. The fastest growth was observed at a salinity below 100 mM NaCl; no growth occurred above 300 mM NaCl. The optimal pH for growth was 6·5; growth was observed from pH 5 to pH 7·5. The G+C content of the genomic DNA was 37·6 mol%. Analysis of the 16S rRNA gene sequence and the partial methyl-CoM reductase gene sequence revealed that the organism was phylogenetically closely related to Methanomethylovorans hollandica DMS1T (98 % similarity for the 16S rRNA gene sequence and 91 % similarity for the methyl-CoM reductase gene sequence). The DNA–DNA relatedness between L2FAWT and Methanomethylovorans hollandica DMS1T was 46 %. On the basis of these results, strain L2FAWT (=DSM 17232T=ATCC BAA-1173T) represents the type strain of a novel species, for which the name Methanomethylovorans thermophila sp. nov. is proposed.

Carbon monoxide conversion by thermophilic sulfate-reducing bacteria in pure culture and in co-culture with Carboxydothermus hydrogenoformans

Biological sulfate (SO(4)) reduction with carbon monoxide (CO) as electron donor was investigated. Four thermophilic SO(4)-reducing bacteria, Desulfotomaculum thermoacetoxidans (DSM 5813), Thermodesulfovibrio yellowstonii (ATCC 51303), Desulfotomaculum kuznetsovii (DSM 6115; VKM B-1805), and Desulfotomaculum thermobenzoicum subsp. thermosyntrophicum (DSM 14055), were studied in pure culture and in co-culture with the thermophilic carboxydotrophic bacterium Carboxydothermus hydrogenoformans (DSM 6008). D. thermoacetoxidans and T. yellowstonii were extremely sensitive to CO: their growth on pyruvate was completely inhibited at CO concentrations above 2% in the gas phase. D. kuznetsovii and D. thermobenzoicum subsp. thermosyntrophicum were less sensitive to CO. In pure culture, D. kuznetsovii and D. thermobenzoicum subsp. thermosyntrophicum were able to grow on CO as the only electron donor and, in particular in the presence of hydrogen/carbon dioxide, at CO concentrations as high as 50-70%. The latter SO(4) reducers coupled CO oxidation to SO(4) reduction, but a large part of the CO was converted to acetate. In co-culture with C. hydrogenoformans, D. kuznetsovii and D. thermobenzoicum subsp. thermosyntrophicum could even grow with 100% CO (P(CO) = 120 kPa).

Metabolic interactions in methanogenic and sulfate-reducing bioreactors

In environments where the amount of electron acceptors is insufficient for complete breakdown of organic matter, methane is formed as the major reduced end product. In such methanogenic environments organic acids are degraded by syntrophic consortia of acetogenic bacteria and methanogenic archaea. Hydrogen consumption by methanogens is essential for acetogenic bacteria to convert organic acids to acetate and hydrogen. Several syntrophic cocultures growing on propionate and butyrate have been described. These syntrophic fatty acid-degrading consortia are affected by the presence of sulfate. When sulfate is present sulfate-reducing bacteria compete with methanogenic archaea for hydrogen and acetate, and with acetogenic bacteria for propionate and butyrate. Sulfate-reducing bacteria easily outcompete methanogens for hydrogen, but the presence of acetate as carbon source may influence the outcome of the competition. By contrast, acetoclastic methanogens can compete reasonably well with acetate-degrading sulfate reducers. Sulfate-reducing bacteria grow much faster on propionate and butyrate than syntrophic consortia.

Reductive decolourisation of azo dyes by mesophilic and thermophilic methanogenic consortia

The contribution of acidogenic bacteria and methanogenic archaea on the reductive decolourisation of azo dyes was assessed in anaerobic granular sludge. Acidogenic bacteria appeared to play an important role in the decolourising processes when glucose was provided as an electron donor; whereas methanogenic archaea showed a minor role when this substrate was supplemented in excess. In the presence of the methanogenic substrates acetate, methanol, hydrogen and formate, methane production became important only after colour was totally removed from the batch assays. This retardation in methane production may be due to either a toxic effect imposed by the azo dyes or to the competitive behaviour of azo dyes to the methanogenic consortia for the available reducing equivalents.

Effect of carbon monoxide, hydrogen and sulfate on thermophilic (55�C) hydrogenogenic carbon monoxide conversion in two anaerobic bioreactor sludges

The conversion routes of carbon monoxide (CO) at 55 degrees C by full-scale grown anaerobic sludges treating paper mill and distillery wastewater were elucidated. Inhibition experiments with 2-bromoethanesulfonate (BES) and vancomycin showed that CO conversion was performed by a hydrogenogenic population and that its products, i.e. hydrogen and CO2, were subsequently used by methanogens, homo-acetogens or sulfate reducers depending on the sludge source and inhibitors supplied. Direct methanogenic CO conversion occurred only at low CO concentrations [partial pressure of CO (PCO) <0.5 bar (1 bar=10(5) Pa)] with the paper mill sludge. The presence of hydrogen decreased the CO conversion rates, but did not prevent the depletion of CO to undetectable levels (<400 ppm). Both sludges showed interesting potential for hydrogen production from CO, especially since after 30 min exposure to 95 degrees C, the production of CH4 at 55 degrees C was negligible. The paper mill sludge was capable of sulfate reduction with hydrogen, tolerating and using high CO concentrations (PCO>1.6 bar), indicating that CO-rich synthesis gas can be used efficiently as an electron donor for biological sulfate reduction.

Interspecies electron transfer in methanogenic propionate degrading consortia

Propionate is a key intermediate in the conversion of complex organic matter under methanogenic conditions. Oxidation of this compound requires obligate syntrophic consortia of acetogenic proton- and bicarbonate reducing bacteria and methanogenic archaea. Although H(2) acts as an electron-carrier in these consortia, evidence accumulates that formate plays an even more important role. To make energy yield from propionate oxidation energetically feasible for the bacteria and archaea involved, the concentrations of H(2) and formate have to be extremely low. On the other hand, the diffusion distance of these carriers has to be small to allow high propionate conversion rates. Accordingly, the high conversion rates observed in methanogenic bioreactors are due to the fact that the propionate-oxidizing bacteria and their methanogenic partners form micro-colonies within the densely packed granules.

Pathways of methanol conversion in a thermophilic anaerobic (55 degrees C) sludge consortium

The pathway of methanol conversion by a thermophilic anaerobic consortium was elucidated by recording the fate of carbon in the presence and absence of bicarbonate and specific inhibitors. Results indicated that about 50% of methanol was directly converted to methane by the methylotrophic methanogens and 50% via the intermediates H(2)/CO(2) and acetate. The deprivation of inorganic carbon species [Sigma(HCO(3)(-)+CO(2))] in a phosphate-buffered system reduced the rate of methanol conversion. This suggests that bicarbonate is required as an electron (H(2)) sink and as a co-substrate for the efficient and complete removal of the chemical oxygen demand. Nuclear magnetic resonance spectroscopy was used to investigate the route of methanol conversion to acetate in bicarbonate-sufficient and bicarbonate-depleted environments. The proportions of [1,2-(13)C]acetate, [1-(13)C]acetate and [2-(13)C]acetate were determined. Methanol was preferentially incorporated into the methyl group of acetate, whereas HCO(3)(-) was the preferred source of the carboxyl group. A small amount of the added H(13)CO(3)(-) was reduced to form the methyl group of acetate and a small amount of the added (13)CH(3)OH was oxidised and found in the carboxyl group of acetate when (13)CH(3)OH was converted. The recovery of [(13)C]carboxyl groups in acetate from (13)CH(3)OH was enhanced in bicarbonate-deprived medium. The small amount of label incorporated in the carboxyl group of acetate when (13)CH(3)OH was converted in the presence of bromoethanesulfonic acid indicates that methanol can be oxidised to CO(2 )prior to acetate formation. These results indicate that methanol is converted through a common pathway (acetyl-CoA), being on the one hand reduced to the methyl group of acetate and on the other hand oxidised to CO(2), with CO(2) being incorporated into the carboxyl group of acetate.

Anaerobic oxidation of 2-chloroethanol under denitrifying conditions by Pseudomonas stutzeri strain JJ

A bacterium that uses 2-chloroethanol as sole energy and carbon source coupled to denitrification was isolated from 1,2-dichloroethane-contaminated soil. Its 16 S rDNA sequence showed 98% similarity with the type strain of Pseudomonas stutzeri (DSM 5190) and the isolate was tentatively identified as Pseudomonas stutzeri strain JJ. Strain JJ oxidized 2-chloroethanol completely to CO(2) with NO(3)(- )or O(2) as electron acceptor, with a preference for O(2) if supplied in combination. Optimum growth on 2-chloroethanol with nitrate occurred at 30 degrees C with a mu(max) of 0.14 h(-1) and a yield of 4.4 g protein per mol 2-chloroethanol metabolized. Under aerobic conditions, the mu(max) was 0.31 h(-1). NO(2)(-) also served as electron acceptor, but reduction of Fe(OH)(3), MnO(2), SO(4)(2-), fumarate or ClO(3)(-) was not observed. Another chlorinated compound used as sole energy and carbon source under aerobic and denitrifying conditions was chloroacetate. Various different bacterial strains, including some closely related Pseudomonas stutzeri strains, were tested for their ability to grow on 2-chloroethanol as sole energy and carbon source under aerobic and denitrifying conditions, respectively. Only three strains, Pseudomonas stutzeri strain LMD 76.42, Pseudomonas putida US2 and Xanthobacter autotrophicus GJ10, grew aerobically on 2-chloroethanol. This is the first report of oxidation of 2-chloroethanol under denitrifying conditions by a pure bacterial culture.

Metabolic interactions between methanogenic consortia and anaerobic respiring bacteria

Most types of anaerobic respiration are able to outcompete methanogenic consortia for common substrates if the respective electron acceptors are present in sufficient amounts. Furthermore, several products or intermediate compounds formed by anaerobic respiring bacteria are toxic to methanogenic consortia. Despite the potentially adverse effects, only few inorganic electron acceptors potentially utilizable for anaerobic respiration have been investigated with respect to negative interactions in anaerobic digesters. In this chapter we review competitive and inhibitory interactions between anaerobic respiring populations and methanogenic consortia in bioreactors. Due to the few studies in anaerobic digesters, many of our discussions are based upon studies of defined cultures or natural ecosystems.

Anaerobic desulphurisation of thiophenes by mixed microbial communities from oilfields

Anaerobic enrichment cultures obtained from oil fields degraded various thiophenic compounds i.e. thiophene, benzothiophene and dibenzothiophene, with the concomitant formation of sulphide using hydrogen, lactate and ethanol as possible electron donors. It was demonstrated that dibenzothiophene was converted to biphenyl. However, hydrocarbon products from benzothiophene and thiophene desulphurisation could not be detected. After further enrichment on thiophenic compounds as the sole electron acceptor, the conversion activity disappeared while homo-acetogenic bacteria became abundantly present. In order to gain stable conversions of thiophenic compounds, attempts were made to isolate the sulphide-producing bacteria. Two highly enriched cultures were obtained, which degraded thiophenic compounds, but the activity remained low and homo-acetogenesis remained dominant.

Image analysis, methanogenic activity measurements, and molecular biological techniques to monitor granular sludge from an EGSB reactor fed with oleic acid

Morphological changes in anaerobic granular sludge fed with increasing loads of oleic acid were quantified by image analysis. The combination of this technique with data on the accumulation of adsorbed long chain fatty acid and with the molecular characterization of microbial community gave insight into the mechanisms of sludge disintegration, flotation and washout. It was found that the bacterial domain was more affected than the archaeal domain during this process. However, no acetoclastic activity and onlya residual hydrogenotrophic activity were detected in the sludge at the end of the operation.

Pseudomonas chloritidismutans sp. nov., a non-denitrifying, chlorate-reducing bacterium

A Gram-negative, facultatively anaerobic, rod-shaped, dissimilatory chlorate-reducing bacterium, strain AW-1(T), was isolated from biomass of an anaerobic chlorate-reducing bioreactor. Phylogenetic analysis of the 16S rDNA sequence showed 100% sequence similarity to Pseudomonas stutzeri DSM 50227 and 98.6% sequence similarity to the type strain of P. stutzeri (DSM 5190(T)). The species P. stutzeri possesses a high degree of genotypic and phenotypic heterogeneity. Therefore, eight genomic groups, termed genomovars, have been proposed based upon deltaTm values, which were used to evaluate the quality of the pairing within heteroduplexes formed by DNA-DNA hybridization. In this study, DNA-DNA hybridization between strain AW-1(T) and P. stutzeri strains DSM 50227 and DSM 5190(T) revealed respectively 80.5 and 56.5% similarity. DNA-DNA hybridization between P. stutzeri strains DSM 50227 and DSM 5190(T) revealed 48.4% similarity. DNA-DNA hybridization indicated that strain AW-1(T) is not related at the species level to the type strain of P. stutzeri. However, strain AW-1(T) and P. stutzeri DSM 50227 are related at the species level. The physiological and biochemical properties of strain AW-1(T) and the two P. stutzeri strains were compared. A common characteristic of P. stutzeri strains is the ability to denitrify. However, in growth experiments, strain AW-1(T) could use only chlorate or oxygen as an electron acceptor and not nitrate, perchlorate or bromate. Strain AW-1(T) is the first chlorate-reducing bacterium described that does not possess another oxyanion-reduction pathway. Cell extracts of strain AW-1(T) showed chlorate and bromate reductase activities but not nitrate reductase activity. P. stutzeri strains DSM 50227 and DSM 5190(T) could use nitrate or oxygen as an electron acceptor, but not chlorate. Chlorate reductase activity, in addition to nitrate reductase activity, was detected in cell extracts of both P. stutzeri strains. Chlorite dismutase activity was absent in extracts of both P. stutzeri strains but was present in extracts of strain AW-1(T). Based on the hybridization experiments and the physiological and biochemical data, it is proposed that strain AW-1(T) be classified as a novel species of Pseudomonas, Pseudomonas chloritidismutans sp. nov. The type strain is strain AW-1(T) (= DSM 13592(T) = ATCC BAA-443(T)).

Hydrogenases and formate dehydrogenases of Syntrophobacter fumaroxidans

The syntrophic propionate-oxidizing bacterium Syntrophobacter fumaroxidans possesses two distinct formate dehydrogenases and at least three distinct hydrogenases. All of these reductases are either loosely membrane-associated or soluble proteins and at least one of the hydrogenases is located in the periplasm. These enzymes were expressed on all growth substrates tested, though the levels of each enzyme showed large variations. These findings suggest that both H2 and formate are involved in the central metabolism of the organism, and that both these compounds may serve as interspecies electron carriers during syntrophic growth on propionate.

Enrichment of thermophilic syntrophic anaerobic glutamate-degrading consortia using a dialysis membrane reactor

A dialysis cultivation system was used to enrich slow-growing moderately thermophilic anaerobic bacteria at high cell densities. Bicarbonate buffered mineral salts medium with 5 mM glutamate as the sole carbon and energy source was used and the incubation temperature was 55 degrees C. The reactor inoculum originated from anaerobic methanogenic granular sludge bed reactors. The microbial population was monitored over a period of 2 years using the most probable number (MPN) technique. In the reactor glutamate was readily degraded to ammonium, methane, and carbon dioxide. Cell numbers of glutamate-degrading organisms increased 400-fold over the first year. In medium supplemented with bromoethane sulfonic acid (BES, an inhibitor of methanogenesis), tenfold lower cell numbers were counted, indicating the syntrophic nature of glutamate degradation. After 2 years of reactor operation the predominant organisms were isolated and characterized. Methanobacterium thermoautotrophicum (strain R43) and a Methanosaeta thermophila strain (strain A) were the predominant hydrogenotrophic and acetoclastic methanogens, respectively. The numbers in which the organisms were present in the reactor after 24 months of incubation were 8.6 x 10(9) and 3.8 x 10(7) mL(-1) sludge, respectively. The most predominant glutamate-degrading organism (8.6 x 10(7) mL(-1) sludge), strain Z, was identified as a new species, Caloramator coolhaasii. It converted glutamate to hydrogen, acetate, some propionate, ammonium, and carbon dioxide. Growth of this syntrophic organism on glutamate was strongly enhanced by the presence of methanogens.

Competition for H2 between sulfate reducers, methanogens and homoacetogens in a gas-lift reactor

Reported values for growth kinetic parameters show an order in competitivity of heterotrophic sulfate reducing bacteria>methanogens>homoacetogens for the substrate hydrogen. This order suggests that methanogens can succesfully compete with consortia of heterotrophic SRB and homoacetogens when H2/CO2 is present as sole substrate. However, we found in experiments using gas-lift reactors inoculated with anaerobic sludge and fed with H2/CO2 and sulfate, that heterotrophic sulfate reduction rapidly and completely outcompeted methanogenesis, whereas a low amount of acetate was formed. Thus, in disagreement with the above competitivity order, hydrogen is more readily consumed by homoacetogenesis than by methanogenesis, indicating that the competition is not kinetically determined. The superior settling velocity of sulfidogenic-acetogenic sludge compared to that of methanogenic sludge suggests that the former sludge is better retained, which can explain the predominance of sulfate reduction/homoacetogenesis over methanogenesis.

The effect of oxygen on methanol oxidation by an obligate methanotrophic bacterium studied by in vivo (13)C nuclear magnetic resonance spectroscopy

13C NMR was used to study the effect of oxygen on methanol oxidation by a type II methanotrophic bacterium isolated from a bioreactor in which methane was used as electron donor for denitrification. Under high (35-25%) oxygen conditions the first step of methanol oxidation to formaldehyde was much faster than the following conversions to formate and carbon dioxide. Due to this the accumulation of formaldehyde led to a poisoning of the cells. A more balanced conversion of (13)C-labelled methanol to carbon dioxide was observed at low (1-5%) oxygen concentrations. In this case, formaldehyde was slowly converted to formate and carbon dioxide. Formaldehyde did not accumulate to inhibitory levels. The oxygen-dependent formation of formaldehyde and formate from methanol is discussed kinetically and thermodynamically.